What Causes Large Rocks to Weather?

Physical Weathering

• The primary source of weathering is nothing more than physical interaction with the environment. This type of erosion includes thermal stress in desert areas, wherein the outer layers of rock become heated and cooled more than the layers beneath them and eventually crack. It also includes frost weathering, wherein water enters cracks in the rock, freezes and expands, deepening the crack in doing so and then sinks further in when it thaws, repeating the process when the temperature drops below freezing again.

Other Types of Physical Weathering

• Pressure release weathering occurs after the removal of a top layer of rock that was previously putting the rock beneath it under stress; the rock beneath it then expands and fractures. Hydraulic action occurs at coastlines, where water enters cracks in the rock and compresses the air in the crack, weakening the rock. This air is then released with tremendous force when the water leaves the rock, causing it to crack further.

Chemical Weathering

• Chemical weathering takes place when water or plant life causes a chemical change in the rock that damages it. Acid rain is a major cause of this phenomenon, as is carbonation. This type of erosion largely occurs in rocks that contain calcium carbonate, such as chalk and limestone, and results from rain reacting with carbon dioxide to form a weak carbonic acid. The carbolic acid in turn reacts with the calcium carbonate to form calcium bicarbonate, which can widen and deepen cracks in calcium carbonate.

Other Types of Chemical Weathering

• Hydration is the chemical weathering process wherein the oxygen and hydrogen that make up water become attached to the molecules and atoms of the mineral. This process takes up more space than the minerals' original chemical structure, which puts it under stress and can cause weathering. Similarly, oxidation occurs when iron-based minerals react with oxygen and water to create oxides and hydroxides such as goethite and hematite. These materials leave a brown coating on the surface of the rock that crumbles easily, damaging it.